The present invention relates to the techniques for reinforcing pipelines and, in particular, to the repair of underground pipelines.
These pipelines are, for example, part of a water supply network. In their commonest form, these pipelines consist of an end-to-end assembly of prestressed concrete pipe segments with a diameter ranging from about 0.5 to 6 meters. Water circulates in them at a pressure which may be about twenty bars. These concrete pipe segments may comprise an inner metal liner. However, the latter is not designed to withstand the pressure forces due to the circulating fluid. These forces are absorbed by the prestressing applied to the concrete by means of wires wound spirally on the outside of the concrete core of the pipe segment. During the prefabrication of such segment, its concrete core is set in rotation about its axis in order to receive the wire which is braked for the purpose of tensioning. This wire is subsequently against corrosion by spraying an additional layer of concrete or mortar over a few tens of millimeters.
In spite of this protecting layer and of the passivation of the steel forming these prestressing wires, it may happens that the prestressing wires experience corrosion which generally leads to a deterioration in the protective coating and therefore to an acceleration of the phenomenon which may cause a break of the wires and therefore a structural embrittlement of the pipeline or even the fracture of the latter under the effect of the pressure of the circulating fluid.
After the damage to one or more segments has been detected by preventive methods (in particular, by acoustic detection), the traditional methods involve first excavating the material (sand, earth or the like) surrounding the damaged segments, then either simply replacing the segments in question, which necessitates draining the supply network beforehand, thus resulting in a considerable detriment to the operation of the network, or manually repairing the outer surface of the pipeline, if the latter has not yet given way.
International Patent Application No. WO 03/014614, the content of which is incorporated herein by reference, discloses a method for repairing a pipeline for the supply of a fluid under pressure, composed of a succession of underground prestressed concrete pipe segments, comprising the steps of:                making a trench in order to free a length of the pipeline;        carrying out a first excavation in the trench under a first pipeline portion substantially centered on a junction between two adjacent segments;        installing and tensioning at least one prestressing reinforcement around the first portion of the pipeline;        filling in the first excavation with soil material and compacting this material;        carrying out a second excavation in the trench under a second pipeline portion adjacent to the first portion and included in a single segment;        installing and tensioning at least one prestressing reinforcement around the second portion of the pipeline; and        filling in the second excavation with soil material and compacting this material.        
This method has the advantage of making it possible to restore a damaged zone of the pipeline, whilst at the same time adhering to the technical constraints associated with this type of structure and its operation.
In some cases, however, there may be problems in making the trench. In particular, the operation of the water supply network may necessitate maintaining a relatively high pressure in the pipeline, for example of the order of 10 bar. The removal of the soil around the pipeline then involves a risk, especially if the damage is serious, since there is no longer the reaction of the soil to oppose the internal pressure. This risk is increased because the work carried out in the trench on the pipeline which has not yet been reinforced might trigger a disastrous fracture.
An object of the present invention is to overcome these limitations by proposing a method making it possible to act on an underground pipeline, in at least some phases of the restoration work, without the need to free it over a considerable length.